When speech or sound signals are transmitted by a packet communication system, a mobile communication system, or the like as represented by Internet communications, compressing and encoding techniques are often used to increase transmission efficiency of the speech or sound signals. Further, in recent years, while encoding speech or sound signals at simply a low bit rate, there is an increasing demand for a technique of encoding speech or sound signals of a broader band.
To meet this need, various techniques have been developed to encode broadband speech or sound signals without substantially increasing the amount of information after encoding. For example, according to a technique disclosed in Patent Literature 1, an encoding apparatus calculates a parameter to generate a spectrum of a high frequency part out of spectrum data obtained by converting an input acoustic signal for a constant time period, and outputs this parameter by matching this with encoded information of a low frequency part. Specifically, the encoding apparatus divides the spectrum data of a high frequency part of a frequency into plurality of sub-bands, and calculates a parameter that specifies a spectrum of a low frequency part that is most similar to the spectrum of each sub-band. Next, the encoding apparatus adjusts the most similar spectrum of a low frequency part by using two kinds of scaling factors such that a peak amplitude, or energy of a sub-band (hereinafter, “sub-band energy”) and a shape in a high-frequency spectrum to be generated becomes similar to a peak amplitude, sub-band energy, and a shape of a spectrum of a high frequency part of an input signal as a target.